1. Field of the Invention
The present invention relates generally to heart rate detection. More particularly, the invention relates to improving heart rate detection accuracy by reducing the influence of physical activity.
2. Background of the Invention
An electrocardiogram (ECG) is a measurement of the electrical signals given off by the beating of the heart. The R waves, which are the noticeable upward spikes on an ECG trace, occur during contraction of the heart and are part of the QRS complex. Thresholding is one method of determining a patient's heart rate, and involves detecting when the R wave amplitude exceeds a predetermined threshold value. However, the real time detection of heart rates is challenging in the presence of amplitude fluctuations that are due to motion.
An example of such a scenario is the detection of R-waves measured either from surface or subcutaneous electrodes placed in the chest area. Respiration induced motion artifact is especially problematic when a patient's breathing is being controlled by a respirator. In this case, exaggerated inflation and deflation of the lungs causes the rise and fall of the chest cavity at a frequency several times lower than the cardiac rate. This rise and fall can cause amplitude fluctuations in the signal measured from the electrodes.
An increase in amplitude can cause detection of portions of the ECG other than the R wave, resulting in spurious beat detections. Conversely, a decrease in amplitude can cause missed detection of R waves, resulting in missed beat detections. Thus, amplitude fluctuations may result in an inaccurate heart rate determination.
Another example is the subcutaneous photoplethysmography (sPPG) sensor, which is extremely sensitive to motion. The real time PPG signal fluctuates greatly with respiration due to motion artifact as well as change in venous blood volume that accompanies each breath.
High-pass filters have been used to remove low-frequency components from the cardiac signal. However, such an approach requires the added complexity of a filter to obtain an accurate heart rate.
What is needed is a more efficient way to detect heart rate in the presence of physical motion such as that due to respiration.